1. Field of the Invention
The present invention relates to a thermally sensitive element whose resistance alters due to changes in the ambient temperature or infrared radiation, and a radiation sensor employing such a thermally sensitive element.
2. Description of the Related Art
Conventionally, temperature sensors employing a thermally sensitive element have been known in which said thermally sensitive element is prepared by carbonizing an organic material such as a phenol resin at 640 to 750.degree. C., as is described in Japanese Patent Laid-Open No. 8-166295. Such temperature sensors detect the changes in resistance, in practice, changes in voltage, from electrodes formed at both ends of the thermally sensitive element based on the fact that the resistance of the thermally sensitive element alters according to the ambient-temperature change.
In addition, a thermally sensitive element composed of a fiber filament essentially consisting of silicon carbide and the like is known to detect infrared rays as a resistance change caused by the temperature change due to absorption of the infrared rays, as is described in Japanese Patent Laid-Open No. 3-96824. For assembling an infrared sensor employing such a thermally sensitive element, two of the four thermally sensitive elements are used for sensing radiation and the other two thermally sensitive elements are employed for temperature compensation, and they are alternately connected in series to form a bridge circuit. Changes in resistance of the thermally sensitive elements due to the ambient temperature can be thereby canceled so that changes in resistance due to only infrared absorption can be detected.
Carbonized materials treated at a low temperature exhibit semiconductor-like conductivity such that the temperature dependency (thermistor constant) increases according to a rise in the specific resistance and decreases according to a reduction in the specific resistance. Meanwhile, for employing such carbonized materials for a thermally sensitive element of a temperature sensor or a radiation sensor, the operating resistance of the thermally sensitive element is preferably low, considering the detecting circuit and level of noise in the sensor. However, in the thermally sensitive element used for the above-mentioned conventional temperature sensors, electrodes are formed on both ends of a sheet of a carbonized material treated at a low temperature so as to detect the resistance along the longitudinal direction of the carbonized sheet material positioned between the electrodes. Thus, to set the operating resistance between the electrodes in a general range, i. e., from several k.OMEGA. to several hundreds k.OMEGA., carbonized materials having high specific resistance cannot be used. Therefore, the temperature dependency of the thermally sensitive element is lowered, thereby disadvantageously reducing the sensor sensitivity.
Although the above thermally sensitive element can be also applied to radiation sensors such as infrared sensors, in such a case it is necessary to have either surface of the thermally sensitive element as a radiation sensitive face receiving radiant rays coming from the outside. However, since the above conventional thermally sensitive elements have their electrodes at both ends thereof, it is impossible to entirely use one surface of the thermally sensitive element as the radiation sensitive face. Thus the whole size of the thermally sensitive elements must be enlarged to ensure the required area of the radiation sensitive face. Therefore, disadvantageously, the heat capacitance of the thermally sensitive elements increases and the response rate of the resulting radiation sensors is lowered.
In a radiation sensor, such as an infrared sensor, employing a thermally sensitive element composed of a fiber filament, when a bridge circuit of the radiation sensor consists of four thermally sensitive elements, these four elements must have the same temperature dependency, which requirement is difficult from a manufacturing aspect. Moreover, electrodes must be fixed to both ends of each fiber filament by a conductive adhesive or the like, thus the assembling procedure is complicated, resulting in high cost.